Upon shutdown of the car, I hear what I think is my brake booster hissing (with hood open, and my head near it). Goes for about 30 seconds. It would not make any sense that if the booster was leaking it would hiss...but Im prepared to be amazed.

Is this normal?

Brakes are a *little* stiff, but that may just be because Im not used to them yet.

Thanks,
dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Sounds like you have a vacuum leak in the booster. The hissing you hear is air leaking into the booster. It stops in 30 seconds as the pressure inside approaches the atmospheric pressure outside.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I guess I was not expecting to hear a hiss from a leak INTO the booster. But it makes sense that it could resonate inside that metal chamber and that woud be what I am hearing.

How can I test this? Could it just be a seal or am I looking at a new booster here?

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

There is an o-ring seal at the base of the master cylinder. That could be your problem. That is an easy/cheap fix. I am not sure how to test it since it is sucking in air. Might be worth just replacing it and seeing if that fixes it.

Get a Mityvac. Disconnect the large line going into the booster AT THE vacuum pump. Disconnect the vacuum lines at the 2 small nipples. Block one of the lines with a golf tee ( leave a small piece of rubber attached . Attach the Mityvac to the other small nipple.
Place your thumb over the disconnected line (vacuum pump connection). Pump the Mityvac until it registers on the gauge. ( yes it will take a lot of pumping) once done let it sit and watch the gauge. If it drops then it is in the booster.
Go to the dealer and get a new master cylinder (MC) to booster Oring ( its actually flat or square) remove the MC and examine the Oring. If bad then you have solved the problem. install it and test it again. If not then it is the internals in the booster and it will have to be replaced. You can reuse the Oring just installed if replacing the booster.
Replacing the booster is a job. Doable but your removing the MC and disconnecting the booster from under the dash. We can post that procedure if you have to replace it.
Replacing the booster is another thread.

Let us know what happens

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

You can actually take out the line from the booster and pump it directly there if you have a fitting that will seal properly. In addition to checking at the resting position, I would also check it a various pedal levels. I found one booster that didn't leak that the resting position, but when you press the pedal the vacuum would leak out fast. And yes you'll need to pump a lot!

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1983 123.133 California
- GreaseCar Veg System

Paul: Maybe Ill run over to the dealer and just snag that part. Lets see how much they want to charge me for it...

Dave: Thanks for the procedure, Ill check it out tonight and report back.

rmarzan - ill check it out at multiple positions. I think itll leak just fine at rest

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-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Tested with mityvac per Daves instructions- the booster leaks vac. The leak is not too fast, maybe a mmHg every 10 seconds or so.

Reconnected the vac line and started the car. Let vac system fill up and turned car off. Hissing is defintely coming from the booster, no doubt about it.

Ill take off the master cylinder tonight to inspect the O-ring., now to figure out how to do that

dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

FWIW, I had a hissing sound that I was convinced was comming from the booster, I replaced the rubber connection from top of IP and everything else rubber too, solved. Good luck.

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83 SD

84 CD

Oreilly's has then a lot cheaper than the dealer. My daughters car has the Oreilly's rebuilt.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car